Sharks are scary. And, frankly, they should be. We humans are scared of sharks, not just because some species have killed people, but because, by definition, if you encounter a shark, you're already in a realm evolutionary biology didn't prepare you for. You are at a disadvantage from the start.
We are well north of 400 species of sharks and counting. New ones are proposed in scientific literature every year, although the last really huge and distinctive shark, requiring a new genus, was the bizarre megamouth (Megachasma pelagios) caught accidentally by a U.S. ship in 1976. The really odd thing about this case was that there were no sightings, strandings, reports, or even folklore hinting at the shark's existence before that date, although the numerous specimens gathered since then indicate there must have been strandings where the shark wasn't identified.
Here in 2014, one shark getting a lot of ink is one that will not be discovered, at least not alive. Megalodon (no relation to megamouth) died out millions of years ago but was brought back, in a sense, by a fake documentary on the Discovery Channel, which used to show only factual science programming and so misled a lot of people despite some rather fuzzy disclaimers. There are some intriguing sighting reports of huge sharks, though most can be blamed on the harmless whale shark, which reaches 15m, and the basking shark, which is almost the same size. One that I still wonder about was that recorded by Dr. D.G. Stead, who questioned witnesses of a sighting in 1918. Lobstermen spoke in awed tones of a ghostly white monstrosity that gobbled meter-wide lobster traps whole like popcorn. Stead noted that some of the estimates of length were "absurd" (one man said 300 feet!) but still felt the men had met "a vast shark." The coloration is almost as much of a mystery as the size in this case.
How many of these species are dangerous to humans? I've seen counts as high as 33 species, but it's hard to sort out: humans are often bitten in circumstances where no one has a good look at the shark or can identify it. Even very small sharks may bite (like most other fish) in the right circumstances. The great white, oceanic whitetip, bull, shortfin mako, and tiger sharks are most often accounted dangerous to humanity. Of these, the whitetip may be the one people hear least about, but it was probably the chief predator in two really horrendous episodes, the predations of sharks after the sinkings of the Nova Scotia and the Indianapolis in World War II.
The blue shark (one of my personal favorites, a beautiful animal and one of the most streamlined creatures in nature) is often feared but rarely fingered as a culprit. Other species like gray reef, nurse, and lemon sharks, while not as large or deadly as their already-mentioned counterparts, figure in many of the "shark bites man" cases.
You are, statistically speaking, unlikely to die of shark bite even if you spend a great deal of time in the oceans. Sharks kill about ten people a year. Crocodiles kill a hundred times as many people. Snakes may kill 5,000 times as many. Even if you leave off the far greater numbers of people killed by disease carrying insects, snails, etc., it's pretty clear sharks rank as amateurs in the human-killing business.
Again, this is about evolutionary biology. Sharks are adapted to search for particular prey. Most of the time a fatal human encounter is precipitated because the shark mistakes a person for a seal or something else on the normal menu. Nonfatal encounters are often the result of a shark biting at something in murky water.
That's not to say there are no shark attacks - incidents where a shark has a good view with its eyes and the rest of its marvelous sensor suite, targets a human, and seriously wounds or kills him or her. Those certainly happen, and it's crazy to approach large sharks casually. The point is that such attacks are freakishly rare in most of the world. The Nova Scotia and Indianapolis episodes were the result of dumping large numbers of humans into the sea where sharks could hardly overlook the buffet thus presented. (The death tolls in each case are known - 852 on the former and 879 on the latter - but the percentage of deaths caused by sharks will always be guesswork. Likewise for the 1943 case of the Cape San Juan, where many of the 825 people who died in the water were shark meals.)
It's common for many sharks to be killed by local authorities when there's a death on the world's beaches. Countless sharks paid for the famous 1916 episode that inspired Jaws (although the perpetrator was most likely a bull shark, assuming it was only one shark.)
Most recently, seven human death in three and a half years caused the government of Western Australia to spent over a million dollars (U.S.) on a program of catching sharks via baited hooks in 2013-4. The catch was reported in June 2014 as 175 sharks. Those over three meters - 50 of them - were deemed dangerous and killed. The cull, however, caught no great whites, the species actually blamed for the recent human deaths. Tiger sharks made up most of the catch. It's not at all clear this controversial effort made any difference to human safety, although it's being extended anyway.
Any human v. shark accounting is a very one-sided affair. It's been authoritatively estimated that humans kill 71 million sharks a year, mostly for the sharkfin soup trade. The number may be higher. The population of sharks obviously can't withstand this onslaught forever. Many nations have moved to protect particular species and block off habitat. There is occasional good news, like the June 2014 report that great whites off the U.S. east coast are much more numerous than previously believed. And not all sharks are in trouble: small deep-water species have no economic importance. However, as long as facilities like this fin-processing factory in China operate, sharks overall are in dire need of increased human assistance.
There remains a great deal of public fascination and fear about sharks. Witness the coverage of a single shark, "Katherine," a 1,000-kilogram-plus great white tagged off Massachusetts and most recently headed for the Gulf Coast of Texas, an event which seemingly has people stirred up as though she was Godzilla.
It's very safe to say we don't know all the shark species of the world yet. It's also, though, clear that many of the best-known species need help. The insanity of the soup trade (especially given that the actual shark fin in a bowl of such soup can almost be measured in individual molecules) and a myriad of other dangers threaten to effectively remove two of the ocean's biggest filter-feeders and several of its apex predators from the food chain. We don't know exactly what the results will be, but they will be serious. They could be catastrophic.
"Sharks are for study, not soup" should be our motto.